Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide method and apparatus for video coding. In some examples, an apparatus includes receiving circuitry and processing circuitry. The processing circuitry decodes prediction information of a chroma coding unit in a segment (such as a slice, a tile, a group of tiles) of a current picture from a coded video bitstream. The segment lacks a temporal reference picture and having separate coding tree structures for luma and chroma components. Then, the processing circuitry detects a prediction mode flag associated with the chroma coding unit from the coded video bitstream, and determines, based on the prediction mode flag, a prediction mode from an intra prediction mode and an intra block copy mode for the chroma coding unit. Further, the processing circuitry reconstructs at least one sample of the chroma coding unit according to the determined prediction mode.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. Ser. No. 16/198,637 filedNov. 21, 2018, which claims the benefit of priority to U.S. ProvisionalApplication No. 62/670,695, “METHODS FOR SIGNALING OF INTRA PICTUREBLOCK COMPENSATION WITH SEPARATE CODING TREE STRUCTURES” filed on May11, 2018, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1, depicted in the lower right is a subset of ninepredictor directions known from H.265's 35 possible predictordirections. The point where the arrows converge (101) represents thesample being predicted. The arrows represent the direction from whichthe sample is being predicted. For example, arrow (102) indicates thatsample (101) is predicted from a sample or samples to the upper right,at a 45 degree angle from the horizontal. Similarly, arrow (103)indicates that sample (101) is predicted from a sample or samples to thelower left of sample (101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEMNVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 2 shows a schematic (201) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

SUMMARY

Aspects of the disclosure provide method and apparatus for video coding.In some examples, an apparatus includes receiving circuitry andprocessing circuitry. The processing circuitry decodes predictioninformation of a chroma coding unit in a segment (such as a slice, atile, a group of tiles) of a current picture from a coded videobitstream. The segment lacks a temporal reference picture and havingseparate coding tree structures for luma and chroma components. Then,the processing circuitry detects a prediction mode flag associated withthe chroma coding unit from the coded video bitstream, and determines,based on the prediction mode flag, a prediction mode from an intraprediction mode and an intra block copy mode for the chroma coding unit.Further, the processing circuitry reconstructs at least one sample ofthe chroma coding unit according to the determined prediction mode.

In some embodiments, the prediction mode flag is used to indicate one ofan inter prediction mode and the intra prediction mode when the segmenthas a temporal reference picture.

According to an aspect of the disclosure, the processing circuitrydetermines the prediction mode to be the intra block copy mode based onthe prediction mode flag, and detects an omission of a specific blockvector for the chroma coding unit by the coded video bitstream. Acombination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates that all co-located luma sub-blocks to the chromacoding unit have the intra block copy mode. In an example, thecombination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates a validity of first block vectors for smallestchroma units in the chroma coding unit that are derived from theco-located luma sub-blocks. For example, the combination of the intrablock copy mode and the omission of the specific block vector for thechroma coding unit by the coded video bitstream indicates that the firstblock vectors that are derived from the co-located luma sub-blocks pointto reference regions that have been decoded in the current picture.

In an embodiment, the processing circuitry then derives the first blockvectors for the smallest chroma units based on second block vectors ofthe co-located luma sub-blocks, and reconstructs the smallest chromaunits based on the first block vectors.

In some embodiments, a combination of the intra block copy mode and theomission of the specific block vector for the chroma coding unit by thecoded video bitstream indicates that all co-located smallest luma unitshave the intra block copy mode. For example, the combination of theintra block copy mode and the omission of the specific block vector forthe chroma coding unit by the coded video bitstream indicates a validityof first block vectors for chroma sub-blocks in the chroma coding unitthat are derived from the co-located smallest luma units.

Thus, in an embodiment, the processing circuitry derives the first blockvectors for the chroma sub-blocks based on second block vectors of theco-located smallest luma units, and reconstructs the chroma sub-blocksbased on the first block vectors.

In some embodiments, the processing circuitry selects the intra blockcopy mode for the chroma coding unit based on the prediction mode flag,and detect a specific block vector for the chroma coding unit from thecoded video bitstream. Then, the processing circuitry reconstructs theat least one sample of the chroma coding unit according to the blockvector.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo coding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

Intra prediction figures

FIG. 1 is a schematic illustration of a subset of intra prediction modesin accordance with some examples.

FIG. 2 is an illustration of intra prediction directions according tosome examples.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure.

FIG. 10 shows an example of a chroma CU and a co-located luma areaaccording to an embodiment of the disclosure.

FIG. 11 shows a flow chart outlining a process example according to anembodiment of the disclosure.

FIG. 12 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (404) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (501), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512) (e.g., a display screen) that is not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler/inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640) (e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5. Brieflyreferring also to FIG. 5, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.256. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEVC example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in FIG. 7.

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (722) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intramode, the general controller (721) controls the switch (726) to selectthe intra mode result for use by the residue calculator (723), andcontrols the entropy encoder (725) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(721) controls the switch (726) to select the inter prediction resultfor use by the residue calculator (723), and controls the entropyencoder (725) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients. In various embodiments, thevideo encoder (703) also includes a residue decoder (728). The residuedecoder (728) is configured to perform inverse-transform, and generatethe decoded residue data. The decoded residue data can be suitably usedby the intra encoder (722) and the inter encoder (730). For example, theinter encoder (730) can generate decoded blocks based on the decodedresidue data and inter prediction information, and the intra encoder(722) can generate decoded blocks based on the decoded residue data andthe intra prediction information. The decoded blocks are suitablyprocessed to generate decoded pictures and the decoded pictures can bebuffered in a memory circuit (not shown) and used as reference picturesin some examples.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (725) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8.

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (872) or the inter decoder (880), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (880); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (872). The residual information can be subject to inversequantization and is provided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603),and (703), and the video decoders (410), (510), and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for intra picture blockcompensation with separate luma and chroma coding trees.

Block based compensation can be used for inter prediction and intraprediction. For the inter prediction, block based compensation from adifferent picture is known as motion compensation. For intra prediction,block based compensation can also be done from a previouslyreconstructed area within the same picture. The block based compensationfrom reconstructed area within the same picture is referred to as intrapicture block compensation, or intra block copy. A displacement vectorthat indicates the offset between the current block and the referenceblock in the same picture is referred to as a block vector (or BV forshort). Different from a motion vector in motion compensation, which canbe at any value (positive or negative, at either x or y direction), ablock vector has a few constraints to ensure that the reference block isavailable and already reconstructed. Also, in some examples, forparallel processing consideration, some reference area that is tileboundary or wavefront ladder shape boundary is excluded.

The coding of a block vector could be either explicit or implicit. Inthe explicit mode, the difference between a block vector and itspredictor is signaled; in the implicit mode, the block vector isrecovered from a predictor (referred to as block vector predictor), in asimilar way as a motion vector in merge mode. The resolution of a blockvector, in some implementations, is restricted to integer positions; inother systems, the block vector is allowed to point to fractionalpositions.

In some examples, the use of intra block copy at block level, can besignaled using a reference index approach. The current picture underdecoding is then treated as a reference picture. In an example, such areference picture is put in the last position of a list of referencepictures. This special reference picture is also managed together withother temporal reference pictures in a buffer, such as decoded picturebuffer (DPB).

There are also some variations for intra block copy, such as flippedintra block copy (the reference block is flipped horizontally orvertically before used to predict current block), or line based intrablock copy (each compensation unit inside an M×N coding block is an M×1or 1×N line).

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure. Current picture (900) is under decoding. The currentpicture (900) includes a reconstructed area (910) (grey area) andto-be-decoded area (920) (white area). A current block (930) is underreconstruction a decoder. The current block (930) can be reconstructedfrom a reference block (940) that is in the reconstructed area (910).The position offset between the reference block (940) and the currentblock (930) is referred to as a block vector (950) (or BV (950)).

Generally, a coding unit includes samples of both luma and chromacomponents. The samples of chroma component can have an independent orseparate split tree structure as compared to the ones of luma component.Typically, such a separate coding tree structure starts from CTU level.Therefore, it is possible that a chroma CU (a CU that contains only twochroma components) can be larger than its luma counterpart at thecorresponding sample location.

In some embodiments, block based motion compensation for all colorcomponents of a block share the same motion or block vector(s). When theseparate coding tree is enabled, the luma-chroma sample correspondenceat the same location may not be always associated. Sometimes a luma CUarea may cover more than one chroma CU. Sometimes, a chroma CU area maycover more than one luma CU.

In some examples, for each chroma CU, its co-located luma area can beused to determine whether the chroma CU should be coded in intra blockcopy mode.

Some aspects of the present disclosure provide methods to properly applythe intra block copy techniques when luma-chroma separate coding treesare used. Specifically, the methods can perform BV derivation of chromablocks in chroma coding tree, when the luma coding tree is separatelycoded ahead of coding of chroma coding tree.

According to an aspect of the disclosure, the property of a chroma CU isderived based on corresponding luma CU's intra block copy information.For each chroma CU, the co-located luma area can be used to determinewhether the current chroma CU should be coded in intra block copy mode.

FIG. 10 shows an example of a chroma CU (1020) and a co-located lumaarea (1010) in 4:2:0 format according to an embodiment of thedisclosure. In the FIG. 10 example, the chroma CU 1020 is an 8×8(samples) Chroma CU and the corresponding luma area (1010). Thecorresponding luma area (1010) includes 16×16 (samples) according to4:2:0 format. In some embodiments, a coding unit is divided intosmallest compensation units for compensation. For example, in the 4:2:0format, the smallest compensation unit for luma has a size of 4×4(samples), and is referred to as a luma unit; the smallest compensationunit for chroma has a size of 2×2 (samples), and is referred to as achroma unit. Thus, when a coding unit is divided into the smallestcompensation units, each smallest chroma unit will have itscorresponding smallest luma unit at the luma-chroma co-located position.Therefore, a chroma CU has its co-located luma area, which is formed ofa set of smallest luma units.

In an example, at an encoder side, when performing intra block copy fora chroma CU, each chroma CU's smallest sub-block (e.g., chroma unit) canbe checked whether its corresponding luma block (e.g., luma unit) iscoded in intra block copy mode. In one method, a chroma CU can be codedin intra block copy mode, only when all the smallest compensation unitsin its corresponding luma area are coded in intra block copy mode. Inthis case, for each smallest block unit in this chroma CU, its BV can bederived from the corresponding smallest luma unit (with proper scaling).This case is referred to as chroma CU has full (intra block copy)coverage in this disclosure. When at least one chroma CU's sub-blockdoes not have its corresponding luma block coded in intra block copymode, this chroma CU does not have the full (intra block copy) coverage.

Aspects of the disclosure provide methods for signaling of intra blockcopy mode in a chroma CU, when the coding of luma coding tree isseparate from chroma coding tree and the coding of the luma coding treeis before the coding of chroma coding tree.

According to an aspect of the disclosure, prediction mode is used forsignaling intra block copy mode. Traditionally, the prediction mode of ablock can be either intra mode or inter mode. Specifically, when asyntax pred_mode_flag for a current block equals to 0, the current blockis coded in the intra mode. Otherwise, pred_mode_flag of the currentblock equals, the current block is coded in the inter mode. However,when a slice in a picture is an intra coded slice, because there is nointer coded blocks in the slice, the pred_mode_flag can be inferred tobe 0 (intra mode). Then, the pre_mode_flag can be used to signal otherinformation, such as intra block copy mode, and the like. Thus, thesemantics of the syntax pred_mode_flag can be determined based on someother information.

When intra block copy mode is used, a few methods are used to signal theintra block copy mode in a slice that does not have temporal referencepicture. In an embodiment, the intra block copy mode is treated as aninter mode, and the current decoded picture is used as a referencepicture. Then for a slice without using temporal reference pictures,blocks can be coded either in intra mode or in intra block copy mode(treated as an inter mode). For example, when the pred_mode_flag for acurrent block equals to 1, the current block is coded in the intra blockcopy mode. Specifically, at a decoder side, when the pred_mode_flag fora block equals 1, the decoder determines that the block is coded ininter mode. Further, when the decoder detects that the slice to whichthe block belong has no temporal reference pictures, the decoderdetermines that the block is coded in the intra block copy mode (whichis treated as inter mode). The decoder may uses the current decodedpicture as the reference picture and then can use reconstructiontechniques for the intra block copy mode (similar to the inter mode) toreconstruct the block.

In another embodiment, the intra block copy mode is treated as a thirdmode, which is different from either intra mode or inter mode. Then fora slice without using temporal reference pictures, blocks can be codedeither in intra mode or in intra block copy mode. In this case, one wayto utilize the same syntax structure is to change the semantics ofpred_mode_flag. For example, when the pred_mode_flag of a current blockequals to 1, the current block is coded in intra block copy mode.Otherwise, when the pred_mode_flag equals to 0, the current block iscoded in intra mode. For slices without using temporal referencepictures, the use of intra block copy mode is inferred by the value ofprediction mode, without explicitly signaling a syntax flag.

It is noted that the above semantics for pred_mode_flag can be swapped(for example 0 means inter mode).

When luma and chroma components have separate coding tree structures ina slice without using temporal reference picture(s), the use of intrablock copy mode for a chroma CU can also be signaled by pred_mode_flag,in a similar way as the above-mentioned method. Thus, a separatesignaling syntax for the use of intra block copy mode in a chroma CU isnot needed. The use of the intra block copy mode can be inferred by thevalue of pred_mode_flag. However, a few conditions should be met inorder to get a decodable bitstream when intra block copy mode is used.

In an embodiment, when the BV(s) of a chroma CU is not signaled, buteach smallest chroma unit's BV is derived from its co-located lumablock's BV, and if the pred_mode_flag is signaled regardless whether achroma CU has full coverage or not, then one of the following bitstreamconformance conditions (a) and (b) should be satisfied:

(a) when luma and chroma components have separate coding treestructures, it is required in the bitstream conformance that theprediction mode for a chroma coding block shall be intra mode, when atleast one chroma sub-block's corresponding luma sub-block is not codedin intra block copy mode, or at least a corresponding luma sub-block iscoded in intra block copy mode but the derived chroma block vector forthis chroma sub-block is not a valid one (pointing to an area thatcannot be used as a reference for current block. For example, an areathat has not yet been reconstructed or not allowed to be used asreference). In an example, at an encoder side, for a chroma coding unitin a slice that does not have temporal reference pictures, when acorresponding luma sub-block for a chroma sub-block is not coded inintra block copy mode, the encoder encodes the chroma coding unit in theintra mode and signals the pre_mode_flag to indicate intra mode for thechroma coding unit. In another example, when a derived chroma blockvector (according to the corresponding luma sub-block based on the intrablock copy mode) for a chroma sub-block is not a valid one, e.g.,pointing to an area of later decoding than the current block), theencoder encodes the chroma coding unit in the intra mode and signals thepre_mode_flag to indicate intra mode for the chroma coding unit.

(b) when luma and chroma components have separate coding treestructures, the bitstream conformance requires that the prediction modefor a chroma coding block can be inter mode (or intra block copy mode)only when all its sub-blocks' corresponding luma sub-blocks are coded inintra block copy mode, and all the derived chroma block vector(s) forthe chroma sub-block(s) are valid (pointing to valid reference area).

Thus, at a decoder side, when the prediction information that is decodedfor a chroma coding unit is indicative of intra block copy mode, thedecoder can derive valid chroma block vectors for chroma sub-blocks inthe chroma coding unit from co-located luma sub-blocks, and then canreconstruct the chroma coding unit using techniques for the intra blockcopy mode.

In another embodiment, when the BV(s) of a chroma CU is not signaled,but each smallest chroma unit's BV is derived from its co-located lumablock's BV, and if the pred_mode_flag is signaled considering chromaCU's coverage, then either of the following (c) and (d) is true:

(c) the pred_mode_flag needs to be signaled only when all the chromaCU's sub-blocks have their corresponding luma smallest units coded inintra block copy mode and have valid BVs;

(d) or in other words, pred_mode_flag will be inferred to 0 (intra mode)if at least one chroma sub-block's corresponding luma sub-block is notcoded in intra block copy mode or does not have valid BV. Thus, in anexample, when a chroma CU does not have full coverage, thepred_mode_flag can be inferred, and does not need to be signaled. At thedecoder side, when the decoder detects that the pred_mode_flag for achroma CU equals 1, the decoder can reconstruct the chroma CU usingtechniques for intra block copy mode. If no pred_mode_flag is signaled,the decoder detects that the chroma CU does not have full coverage, andinfers that the pre_mode_flag equals one. Then the decoder canreconstruct the chroma CU using techniques for intra mode.

In another embodiment, when the BV(s) of a chroma CU is not signaled,and each smallest chroma unit's BV is derived from its collocated lumablock's BV, and the syntax pred_mode_flag is not signaled for followingconditions (e) and (f):

(e) if the chroma CU has full coverage and each derived chroma BV isvalid (referred to reconstructed, available reference area outsidecurrent chroma CU), then the syntax pred_mode_flag is not signaled forthe chroma CU. It is inferred to intra block copy mode. At the decoderside, the decoder can determine that the condition (e) is satisfied, theprediction mode is intra block copy mode, and then can reconstruct thechroma CU using techniques for intra block copy mode.

(f) If the chroma CU has does not full coverage, or it has full coveragebut at least one derived chroma BV is not valid, then the syntaxpred_mode_flag is not signaled for the chroma CU. It is inferred tointra mode. At the decoder side, the decoder can determine thatcondition (f) is satisfied, the prediction mode is intra mode, and thencan reconstruct the chroma CU using techniques for intra mode.

According to another aspect of the disclosure, additional syntax flagcan be used to signal the intra block copy mode. In some embodiments, ifthe intra block copy mode is considered as an intra mode, then afterparsing the pred_mode_flag as intra mode, another syntax flag (e.g.,ibc_flag) is needed to tell whether current block is coded regular intramode or intra block copy mode. For signal this ibc_flag flag for chromaCU with luma-chroma components having separate splitting treestructures, the above discussed conditions also apply to it. In anembodiment, the ibc_flag is signaled regardless the chroma CU coverage,but the value of ibc_flag can be true (use intra block copy mode) onlywhen chroma CU has full coverage and all its derived chroma BVs arevalid. In another embodiment, ibc_flag is signaled only when the chromaCU has full coverage, but its value can be true only when all itsderived chroma BVs are valid. In another embodiment, ibc_flag is notsignaled. It is inferred to be true when chroma CU has full coverage andall its derived chroma BVs are valid; otherwise it is inferred to befalse.

According to another aspect of the disclosure, one of intra predictionmodes is used for signaling intra block copy mode.

If the intra block copy mode is treated as an intra-picture coding mode,then the prediction mode for intra block copy can also be intra mode. Ifa chroma CU and a luma CU have separate coding structures, theindication of use of intra block copy mode can be signaled using one ofthe intra prediction mode indices, instead of using prediction mode asabove. In this case, the prediction mode in such a slice (without usingtemporal reference pictures) is intra mode.

In one embodiment, the last index of the intra prediction mode list ismodified to indicate the intra block copy mode. Thus, a chroma CU, ifcoded in the intra block copy mode, is indicated by using the last indexof intra prediction mode list. In this case, the number of intraprediction modes for a chroma CU remains unchanged.

In another embodiment, an additional index is added into the intraprediction mode list behind the last index of the intra prediction modelist to generate a modified intra prediction mode list. Then, a chromaCU, if coded in intra block copy mode, is indicated by using theadditional index which is the last index of modified intra predictionmode list. In this case, the number of intra prediction modes for achroma CU is added by one.

FIG. 11 shows a flow chart outlining a process (1100) according to anembodiment of the disclosure. The process (1100) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1100) are executed by processing circuitry,such as the processing circuitry in the terminal devices (310), (320),(330) and (340), the processing circuitry that performs functions of thevideo encoder (403), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the video decoder (510), the processing circuitry thatperforms functions of the intra prediction module (552), the processingcircuitry that performs functions of the video encoder (603), theprocessing circuitry that performs functions of the predictor (635), theprocessing circuitry that performs functions of the intra encoder (722),the processing circuitry that performs functions of the intra decoder(872), and the like. In some embodiments, the process (1100) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1100). The process starts at (S1101) and proceeds to(S1110).

At (S1110), prediction information of a chroma coding unit is decodedfrom a coded video bitstream. In some examples, the chroma unit is in asegment (such as a slice, a tile, a group of tiles, and the like) of acurrent picture, and the segment lacks a temporal reference picture. Theprediction information also indicates that separate coding treestructures for luma and chroma components are used in the coded videobitstream.

At (S1120), a prediction mode flag associated with the chroma codingunit is detected from the coded video bitstream.

At (S1130) a prediction mode is determined, based on the prediction modeflag. In some examples, due to the lack of temporal reference picturesby the segment, the prediction mode can be an intra prediction mode oran intra block copy mode. Then, based on the prediction mode flag, theprediction mode for the chroma coding unit can be selected from theintra prediction mode (intra mode) and the intra block copy mode.

At (S1140), samples of the chroma coding unit are reconstructedaccording to the determined prediction mode. Then, the process proceedsto (S1199) and terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 12 shows a computersystem (1200) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 12 for computer system (1200) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1200).

Computer system (1200) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1201), mouse (1202), trackpad (1203), touchscreen (1210), data-glove (not shown), joystick (1205), microphone(1206), scanner (1207), camera (1208).

Computer system (1200) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1210), data-glove (not shown), or joystick (1205), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1209), headphones(not depicted)), visual output devices (such as screens (1210) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1200) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1220) with CD/DVD or the like media (1221), thumb-drive (1222),removable hard drive or solid state drive (1223), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1200) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1249) (such as, for example USB ports of thecomputer system (1200)); others are commonly integrated into the core ofthe computer system (1200) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1200) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1240) of thecomputer system (1200).

The core (1240) can include one or more Central Processing Units (CPU)(1241), Graphics Processing Units (GPU) (1242), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1243), hardware accelerators for certain tasks (1244), and so forth.These devices, along with Read-only memory (ROM) (1245), Random-accessmemory (1246), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1247), may be connectedthrough a system bus (1248). In some computer systems, the system bus(1248) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1248),or through a peripheral bus (1249). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1241), GPUs (1242), FPGAs (1243), and accelerators (1244) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1245) or RAM (1246). Transitional data can be also be stored in RAM(1246), whereas permanent data can be stored for example, in theinternal mass storage (1247). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1241), GPU (1242), massstorage (1247), ROM (1245), RAM (1246), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1200), and specifically the core (1240) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1240) that are of non-transitorynature, such as core-internal mass storage (1247) or ROM (1245). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1240). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1240) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1246) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1244)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

JEM: joint exploration model

VVC: versatile video coding

BMS: benchmark set

MV: Motion Vector

HEVC: High Efficiency Video Coding

SEI: Supplementary Enhancement Information

VUI: Video Usability Information

GOPs: Groups of Pictures

TUs: Transform Units,

PUs: Prediction Units

CTUs: Coding Tree Units

CTBs: Coding Tree Blocks

PBs: Prediction Blocks

HRD: Hypothetical Reference Decoder

SNR: Signal Noise Ratio

CPUs: Central Processing Units

GPUs: Graphics Processing Units

CRT: Cathode Ray Tube

LCD: Liquid-Crystal Display

OLED: Organic Light-Emitting Diode

CD: Compact Disc

DVD: Digital Video Disc

ROM: Read-Only Memory

RAM: Random Access Memory

ASIC: Application-Specific Integrated Circuit

PLD: Programmable Logic Device

LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution

CANBus: Controller Area Network Bus

USB: Universal Serial Bus

PCI: Peripheral Component Interconnect

FPGA: Field Programmable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding, comprising: decodingprediction information of a chroma coding unit in a segment of a currentpicture from a coded video bitstream, the segment having separate codingtree structures for luma and chroma components; detecting a predictionmode flag associated with the chroma coding unit from the coded videobitstream, the prediction mode flag being one of two values, a firstvalue of the prediction mode flag indicating a prediction mode for thechroma coding unit is one of an intra block copy mode and an interprediction mode for the chroma coding unit and a second value of theprediction mode flag indicating the prediction mode for the chromacoding unit is an intra prediction mode; determining, when theprediction mode flag is the first value of the two values predictionmode flag, which of the intra block copy mode and the inter predictionmode is the prediction mode indicated by the first value of theprediction mode flag for the chroma coding unit; and reconstructing atleast one sample of the chroma coding unit according to the determinedprediction mode.
 2. The method of claim 1, wherein the determiningcomprises: determining which of the intra block copy mode and the interprediction mode is the prediction mode indicated by the first value ofthe prediction mode flag based on whether the segment has a temporalreference picture.
 3. The method of claim 1, further comprising:detecting an omission of a specific block vector for the chroma codingunit by the coded video bitstream, wherein a combination of the intrablock copy mode and the omission of the specific block vector for thechroma coding unit by the coded video bitstream indicates that apredetermined set of luma blocks is all co-located luma sub-blocks tothe chroma coding unit.
 4. The method of claim 3, wherein thecombination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates a validity of first block vectors for smallestchroma units in the chroma coding unit that are derived from theco-located luma sub-blocks.
 5. The method of claim 4, wherein thecombination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates that the first block vectors that are derived fromthe co-located luma sub-blocks point to reference regions that have beendecoded in the current picture.
 6. The method of claim 4, furthercomprising: deriving the first block vectors for the smallest chromaunits based on second block vectors of the co-located luma sub-blocks;and reconstructing the smallest chroma units based on the first blockvectors.
 7. The method of claim 1, further comprising: detecting anomission of a specific block vector for the chroma coding unit by thecoded video bitstream, wherein a combination of the intra block copymode and the omission of the specific block vector for the chroma codingunit by the coded video bitstream indicates that a predetermined set ofluma blocks is all co-located smallest luma units.
 8. The method ofclaim 7, wherein the combination of the intra block copy mode and theomission of the specific block vector for the chroma coding unit by thecoded video bitstream indicates a validity of first block vectors forchroma sub-blocks in the chroma coding unit that are derived from theco-located smallest luma units.
 9. The method of claim 8, furthercomprising: deriving the first block vectors for the chroma sub-blocksbased on second block vectors of the co-located smallest luma units; andreconstructing the chroma sub-blocks based on the first block vectors.10. The method of claim 1, further comprising: selecting the intra blockcopy mode for the chroma coding unit based on the prediction mode flag;detecting a specific block vector for the chroma coding unit from thecoded video bitstream; and reconstructing the at least one sample of thechroma coding unit according to the block vector.
 11. An apparatus forvideo decoding, comprising: processing circuitry configured to: decodeprediction information of a chroma coding unit in a segment of a currentpicture from a coded video bitstream, the segment having separate codingtree structures for luma and chroma components; detect a prediction modeflag associated with the chroma coding unit from the coded videobitstream, the prediction mode flag being one of two values, a firstvalue of the prediction mode flag indicating a prediction mode for thechroma coding unit is one of an intra block copy mode and an interprediction mode for the chroma coding unit and a second value of theprediction mode flag indicating the prediction mode for the chromacoding unit is an intra prediction mode; determine, when the predictionmode flag is the first value of the two values prediction mode flag,which of the intra block copy mode and the inter prediction mode is theprediction mode indicated by the first value of the prediction mode flagfor the chroma coding unit; and reconstruct at least one sample of thechroma coding unit according to the determined prediction mode.
 12. Theapparatus of claim 11, wherein the processing circuitry is configuredto: determine which of the intra block copy mode and the interprediction mode is the prediction mode indicated by the first value ofthe prediction mode flag based on whether the segment has a temporalreference picture.
 13. The apparatus of claim 11, wherein the processingcircuitry is configured to: detect an omission of a specific blockvector for the chroma coding unit by the coded video bitstream, whereina combination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates that a predetermined set of luma blocks is allco-located luma sub-blocks to the chroma coding unit.
 14. The apparatusof claim 13, wherein the combination of the intra block copy mode andthe omission of the specific block vector for the chroma coding unit bythe coded video bitstream indicates that first block vectors that arederived from the co-located luma sub-blocks point to reference regionsthat have been decoded in the current picture.
 15. The apparatus ofclaim 14, wherein the processing circuitry is configured to: derive thefirst block vectors for the smallest chroma units based on second blockvectors of the co-located luma sub-blocks; and reconstruct the smallestchroma units based on the first block vectors.
 16. The apparatus ofclaim 11, wherein the processing circuitry is configured to: determinethe prediction mode to be the intra block copy mode based on theprediction mode flag; and detect an omission of a specific block vectorfor the chroma coding unit by the coded video bitstream, wherein acombination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates that a predetermined set of luma blocks is allco-located smallest luma units.
 17. The apparatus of claim 16, whereinthe combination of the intra block copy mode and the omission of thespecific block vector for the chroma coding unit by the coded videobitstream indicates a validity of first block vectors for chromasub-blocks in the chroma coding unit that are derived from theco-located smallest luma units.
 18. The apparatus of claim 17, whereinthe processing circuitry is configured to: derive the first blockvectors for the chroma sub-blocks based on second block vectors of theco-located smallest luma units; and reconstruct the chroma sub-blocksbased on the first block vectors.
 19. The apparatus of claim 12, whereinthe processing circuitry is configured to: select the intra block copymode for the chroma coding unit based on the prediction mode flag;detect a specific block vector for the chroma coding unit from the codedvideo bitstream; and reconstruct the at least one sample of the chromacoding unit according to the block vector.
 20. A non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform: decodingprediction information of a chroma coding unit in a segment of a currentpicture from a coded video bitstream, the segment having separate codingtree structures for luma and chroma components; detecting a predictionmode flag associated with the chroma coding unit from the coded videobitstream, the prediction mode flag being one of two values, a firstvalue of the prediction mode flag indicating a prediction mode for thechroma coding unit is one of an intra block copy mode and an interprediction mode for the chroma coding unit and a second value of theprediction mode flag indicating the prediction mode for the chromacoding unit is an intra prediction mode; determining, when theprediction mode flag is the first value of the two values predictionmode flag, which of the intra block copy mode and the inter predictionmode is the prediction mode indicated by the first value of theprediction mode flag for the chroma coding unit; and reconstructing atleast one sample of the chroma coding unit according to the determinedprediction mode.